Arrow consumption script for macro

This is for ad&d 2e, but can be adapted I think for any game macro.

This is what I have so far.
Arrows left: [[(@{arrows}-1)]]
Arrows left: [[(@{arrows}-2)]]
Arrows left: [[(@{arrows}-3)]]

Built a macro for PBS/R that fires (rolls) 3 atks at once. It counts down from total {arrows} attribute. Displays in chat perfectly. But I cant figure out how to get it to overwrite the 1st box of attribute when it is finished. EG: Subtract 3 arrows from x/36, and rewrite x value. Whenever I use it now, it counts down from 36, but doesn't overwrite the original number. Any ideas?

And yes, after I build this, for end of encounter will be creating a macro for arrow-recovery that needs to count the total consumed, and attempt to reclaim x value with con for save. vs crush (like gars arrows recovery, but intead of X rounds 1d20, it would look at total used in encounter and attempt to recover x consumed arrows, then add that back to 1st box of arrows attribute after rate of 1d4/rd recovery}.

You (or rather, the creator of the game) needs to be a Pro subscriber and install an API script to be able to modify attributes via macros. The canonical choices would be Ammo or ChatSetAttr.

Check out: https://wiki.roll20.net/API:Introduction

Ty Silvyre